Two-stage conversion treatment

ABSTRACT

The invention relates to a chromium-free, corrosion prevention treatment for steel, zinc or zinc alloys, aluminum, magnesium or alloys thereof in which the metal surfaces are contacted in a first step with a first aqueous solution having a pH of 1.5 to 5 which contains Ti and/or Zr and/or Si ions and at least such a quantity of fluoride that the atomic ratio of Ti to F and/or Zr to F and/or Si to F is in the range from 1:1 to 1:6 and then, optionally after rinsing with water, and thereafter in a second step are contacted with a second aqueous solution having a pH of 1.5 to 5 which contains Ti and/or Zr and/or Si ions and at least such a quantity of fluoride that the atomic ratio of Ti to F and/or Zr to F and/or Si to F is in the range from 1:1 to 1:6, and which additionally contains such a quantity of soluble anions of oxo acids of Mo (VI) and/or W (VI) that the total concentration of molybdenum and/or tungsten is in the range from 5 to 1500 mg/l.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 USC Sections 365(c) and 120 of International Application No. PCT/EP2004/012323, filed 30 Oct. 2004 and published 7 Jul. 2005 as WO 2005/061761, which claims priority from German Application No. 103 58 310.6, filed 11 Dec. 2003, each of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates generally to the anti-corrosion treatment of metal surfaces, for example of vehicle bodies or domestic appliances. In this treatment, a corrosion-inhibiting layer is produced on iron-containing surfaces, such as steel surfaces for example, and/or on selected nonferrous surfaces, such as for example zinc or zinc alloys (for example galvanized or alloy-galvanized steel), aluminum, magnesium or alloys thereof. This corrosion-inhibiting layer improves protection against corrosion and the adhesion of a subsequently applied layer based on organic polymers, such as a paint or an adhesive for example. A particular feature of the invention in this regard is that no toxic chromium has to be used.

BACKGROUND OF THE INVENTION

Structural elements fitted together from metal sheets, such as vehicle bodies, housings of domestic appliances or metal furniture items, can be assembled from metal sheets which do not yet have a permanent corrosion-inhibiting coating. In a sequence of several process steps, a permanent corrosion-inhibiting coating consisting of a conversion layer and a paint layer can be produced after the metal parts have been assembled. A known example of this is the process sequence of phosphating and painting which is normally applied, for example, in car manufacture. The term “conversion treatment” infers that components of the treatment solution react chemically with the metal surface, resulting in the formation of a corrosion-preventing layer in which both components of the treatment solution and metal atoms from the metal surface are incorporated.

At present, vehicle bodies, such as car bodies for example, are assembled from steel and/or other metallic materials, such as galvanized steel or aluminum for example. After assembly, the bodies are cleaned and are subjected before painting to a conversion treatment to obtain adequate corrosion protection and adequate paint adhesion. The bodies are then painted, nowadays usually by cathodic dipping. Domestic appliances containing metallic components, for example refrigerators, deep freezers, washing machines, spin dryers, cookers, microwave ovens, and even metal furniture can be subjected to a similar sequence of process steps. In view of the less stringent requirements which the corrosion prevention of such articles has to satisfy, the articles are generally painted with a power coating after the conversion treatment.

Phosphating is widely used as a conversion treatment for domestic appliances. In the case of vehicle bodies, the conversion treatment is carried out exclusively as so-called “layer-forming” zinc phosphating. To this end, the vehicle bodies are contacted with an aqueous solution having a pH of about 2.5 to about 3.8 which contains about 0.3 to 2 g/l zinc ions and about 10 to about 20 g/l phosphate ions. In many cases, the phosphating solutions additionally contain about 0.3 to 2 g/l manganese ions and, often, nickel or copper ions. In this treatment, a layer of crystalline zinc-iron phosphates is formed on steel surfaces while a layer of crystalline zinc phosphates is formed on zinc or aluminum surfaces.

To enable these crystalline zinc-containing phosphate layers to develop an adequate effect in regard to corrosion protection and paint adhesion, the actual phosphating step is accompanied by additional steps. For example, the metal surfaces are first cleaned and then activated—generally in several stages—before the phosphating step. For the activation step, the metal surfaces are contacted with a solution which mainly contains secondary alkali metal phosphates and suspended colloidal titanium phosphates. This step has to be carefully monitored to guarantee satisfactory quality of the subsequent phosphating step. In particular, the activating baths are exhausted relatively quickly compared with phosphating baths, so that they have to be renewed at short intervals of a few days to a few weeks. Accordingly, the monitoring and management of the activating baths represent a significant part of the cost of managing and monitoring a phosphating line.

The actual phosphating step is generally followed by a so-called after-passivation. This after-passivation closes any pores remaining in the crystalline phosphate layer and improves corrosion protection and paint adhesion. To this end, the phosphated metal surfaces are contacted with an aqueous solution which may contain various components. After-passivation solutions based on hexavalent chromium, complex fluorides of titanium and/or zirconium, reactive polymers of vinyl phenol derivatives or even on copper ions are in use at the present time. These after-passivation baths also have to be regularly checked and replenished.

Accordingly, besides cleaning, a conversion treatment in the form of phosphating generally requires at least three treatment baths for activation, phosphating and after-passivation, which all have to be regularly monitored and replenished or renewed as necessary. These at least three necessary baths and the rinsing baths additionally present between them lead to a high space requirement and capital investment and thus increase the cost of manufacturing vehicle bodies and domestic appliances. In addition, waste containing heavy metals is formed during the phosphating step and has to be expensively disposed of.

Besides phosphating, there are other known processes for producing a so-called conversion layer which protects the underlying metal against corrosion and which acts as a primer for a subsequent layer of paint. This can be achieved, for example, with conversion solutions based on complex fluorides of boron, silicon, titanium or zirconium. These complex fluorides are mostly used together with organic polymers. Examples of such conversion treatments can be found in DE-A-101 31 723 and the literature cited therein. So far, however, none of these alternative processes has been able to displace phosphating as a pretreatment before painting in car manufacture. The same also applies to the teachings of the following three documents:

U.S. Pat. No. 6,193,815 describes a treatment solution for producing a conversion layer on bare metal surfaces which contains the following components:

-   -   a) 0.01 to 5 parts by weight of dissolved phosphate ions,     -   b) 0.1 to 2 parts by weight of titanium ions,     -   c) 0.05 to 5 parts by weight of fluoride ions and     -   d) 0.01 to 2 parts by weight of a water-soluble accelerator         which may be, for example, a combination of nitric acid and         ammonium heptamolybdate.

It is clear that the titanium ions and the fluoride ions react with one another to form fluoro complexes or may be directly used as fluoro complexes. According to the Examples, not only heptamolybdate, but also tungstate may be used as an accelerator. This treatment solution is particularly suitable for the treatment of aluminum surfaces.

WO 03/078682 discloses a method for producing a conversion layer on a metallic surface by treatment with an aqueous solution containing the following components: a) a source of tungstate ions and b) a soluble material containing zirconium. The surface is then dried and/or baked. Suitable zirconium compounds are said to be, for example, hexafluorozirconic acid and salts thereof.

U.S. Pat. No. 5,449,415 describes a “no-rinse” conversion process for—in particular—cold-rolled steel. “No-rinse” means that the treatment solution is not rinsed off after application, but instead is directly dried. The treatment solution contains as essential components

-   -   a) an anionic component which may be, for example, a fluoro         complex of titanium or zirconium,     -   b) a cationic component selected from the metals Co, Mg, Mn, Zn,         Ni, Sn, Cu, Zr, Fe and Sr,     -   c) a sufficient quantity of acid to adjust a pH of 0.5 to 5,     -   d) oxo anions of phosphorus or phosphonate anions,     -   e) an organic polymer.

Preferably, this solution additionally contains another component selected from tungstate, molybdate, silicotungstate and silicomolybdate.

The above-mentioned examples of conversion solutions, which contain complex fluorides of titanium or zirconium, represent conversion solutions for producing conversion layers on bare metal surfaces. However, it is also known that solutions of complex fluorides of titanium or zirconium can be used for the passivating after-treatment of phosphate layers.

BRIEF SUMMARY OF THE INVENTION

The problem addressed by the present invention was to make the process chain involved in car manufacture, for example, more economical and ecological. The process according to the invention has the following, and other, objects: Avoiding the formation of waste containing heavy metals, more particularly toxic heavy metals, such as chromium or nickel; shortening the process chain, i.e. steps, in relation to conventional phosphating; shortening the physical length of the production line and reducing the number of necessary rinsing steps, thereby saving water. Desirably the foregoing objects are achieved and the conversion layer produced matches a conventional phosphating layer in its quality in regard to corrosion protection and paint adhesion.

One object of the invention is to provide a process for the two-step corrosion prevention treatment of metal surfaces, in which the metal surfaces are contacted in a first step with a chromium-free first aqueous solution having a pH of 1.5 to 5 which contains in all at least 0.01 g/l, preferably at least 0.025 g/l and up to 10 g/l, preferably up to 1 g/l and more particularly up to 0.5 g/l Ti and/or Zr and/or Si ions and at least such a quantity of fluoride that the atomic ratio of Ti to F and/or Zr to F and/or Si to F is in the range from 1:1 to 1:6 and then, optionally after rinsing with water, and thereafter in a second step are contacted with a chromium-free second aqueous solution having a pH of 1.5 to 5 which contains in all at least 0.01 g/l, preferably at least 0.025 g/l and up to 10 g/l, preferably up to 1 g/l and more particularly up to 0.5 g/l Ti and/or Zr and/or Si ions and at least such a quantity of fluoride that the atomic ratio of Ti to F and/or Zr to F and/or Si to F is in the range from 1:1 to 1:6, and which additionally contains such a quantity of soluble anions of oxo acids of molybdenum and/or tungsten with the oxidation number VI that the total concentration of molybdenum and/or tungsten, expressed as MoO₂ ²⁻ and/or WO₄ ²⁻, is in the range from 5 to 1500 mg/l.

Another object of the invention is to provide a process for producing a structural element containing painted metal parts in which I) sheets of metal carrying a coating based on organic polymers are cut and/or stamped and/or formed and the metal parts obtained are fitted together to form the structural element, areas of the metal surface of the sheet which are not covered by the coating based on organic polymers being formed; II) the assembled structural element is cleaned; III) the cleaned, assembled structural element is coated with a passivation layer that is not a zinc phosphate layer on those areas of the metal surface formed in step I) which are not covered by the coating based on organic polymers, the areas of the metal surface of the structural element which are not covered by the coating based on organic polymers are then contacted with the chromium-free first aqueous solution and chromium-free second aqueous solution as described above; IV) if desired, the structural element is then rinsed one or more times with water, although this is not essential, and V) is coated with a layer of paint.

It is a further object of the invention to provide coatings and a process for depositing them, as described herein, for metal surfaces selected from surfaces of steel, galvanized or alloy-galvanized steel, aluminized steel, zinc, aluminium, magnesium or alloys of which at least 50 atom-% consists of zinc, aluminium or magnesium.

It is also an object of the invention, to provide a first aqueous solution that additionally contains at least 0.005 g/l, preferably at least 0.01 g/l and up to 20 g/l, preferably up to 1 g/l organic polymers. It is a further object of the invention, to provide a second aqueous solution that contains no more than 5 mg/l organic polymers. The second aqueous solution may also contain complexing agents.

An object of the invention with regard to pH is to provide a process wherein the first and/or the second aqueous solution has a pH of at least 1.5, preferably of at least 1.7 and up to 5, preferably up to 4. In a yet further object of the invention, the second aqueous solution additionally contains buffer substances for the pH range of 1.5 to 5.

Another object of the invention is to provide a process characterized in that, in the second step b), the metal surfaces are contacted with the second aqueous solution for 5 seconds to 10 minutes and preferably for 10 seconds to one minute. In a yet further object of the invention, the second aqueous solution has a temperature of at least 20° C., preferably of at least 30° C and no more than 60° C., preferably no more than 50° C.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process for the two-step corrosion prevention treatment of metal surfaces in which the metal surfaces

-   -   a) are contacted in a first step with a chromium-free first         aqueous solution having a pH of 1.5 to 5 which contains in all         at least 0.01 g/l, preferably at least 0.025 g/l and up to 10         g/l, preferably up to 1 g/l and more particularly up to 0.5 g/l         Ti and/or Zr and/or Si ions and at least such a quantity of         fluoride that the atomic ratio of Ti to F and/or Zr to F and/or         Si to F is in the range from 1:1 to 1:6 and then, optionally         after rinsing with water,     -   b) are contacted in a second step with a chromium-free second         aqueous solution having a pH of 1.5 to 5 which contains in all         at least 0.01 g/l, preferably at least 0.025 g/l and up to 10         g/l, preferably up to 1 g/l and more particularly up to 0.5 g/l         Ti and/or Zr and/or Si ions and at least such a quantity of         fluoride that the atomic ratio of Ti to F and/or Zr to F and/or         Si to F is in the range from 1:1 to 1:6, and which additionally         contains such a quantity of soluble anions of oxo acids of         molybdenum and/or tungsten with the oxidation number VI that the         total concentration of molybdenum and/or tungsten, expressed as         MoO₂ ²⁻ and/or WO₄ ²⁻, is in the range from 5 to 1500 mg/l.

This two-stage process is of course part of the longer treatment chain involved in the production of painted metal articles such as, for example, car bodies, domestic appliances and the like. In this treatment chain, the parts fitted together are first cleaned and rinsed before being subjected to the two-stage conversion treatment according to the invention. Between the two steps a) and b) of the two-stage treatment according to the invention, the articles may be rinsed with water although this is not essential. After the second step b), the articles are generally rinsed one or more times with water, the final rinsing step preferably being carried out with deionized water. The article may then be painted. Where the requirements which corrosion protection is expected to meet are not strict, as is the case with domestic appliances for example, painting may be carried out with a powder coating. In order to satisfy the more stringent corrosion prevention requirements of car manufacture, the usual multilayer paint coating is applied, the layer next to the metal normally being a cathodic dipping paint.

The process according to the invention is not only capable of preparing the metal surfaces for painting, it may also serve as a basis for bonding. In that case, steps a) and b) are the only steps (apart from the usual rinsing steps) which have to be carried out between cleaning of the parts and application of a layer based on organic polymers, such as a paint or an adhesive for example, for the conversion treatment of the metal surfaces. There is no need for additional activation, as in phosphating for example, or for another after-passivation step.

The two-stage process according to the invention may also be used for the passivation of bare metal areas which are formed, for example, when the parts, such as car bodies for example, are assembled entirely from already precoated material. In this case, the following considerations apply:

In principle, it would be economically and ecologically advantageous to produce metal parts from material already precoated by the manufacturer of the metal sheets and simply to clean and paint those parts after assembly. Waste associated with the pretreatment would thus accumulate in a single location, i.e. at the manufacturer of the metal sheets, and not more widely at the further processors of the metal sheets. Precoated metal sheets would thus be directly available on the market. On the one hand, they could be pre-phosphated, i.e. could carry a phosphate layer, but no other coating based on organic polymers. Metal sheets already provided with a corrosion-inhibiting layer by the manufacturer are also being processed to an increasing extent in the automotive and domestic appliance industries. Corresponding materials are known, for example, under the names of Granocoat®, Durasteel®, Bonazinc® and Durazinc®. They carry a thin organic coating over a conversion layer, for example a chromating or phosphating layer. The organic coating consists of polymer systems such as, for example, epoxy or polyurethane resins, polyamides and polyacrylates. Solid additives, such as silicas, zinc dust and carbon black, improve corrosion protection and, by virtue of their electrical conductivity, enable the metal parts coated with layers about 0.3 to about 10 μm and preferably up to about 5 μm thick to be electrically welded and electrolytically painted. The substrate materials are generally coated in a two-stage process, in which the inorganic conversion layer is first produced and the organic polymer film is then applied in a second treatment stage. More information on this process can be found in DE-A-100 22 075 and the literature cited therein.

Accordingly, metal sheets provided with a coating based on organic polymers by coil coating are already partly in use in the manufacture of vehicle bodies, domestic appliances and items of furniture. In car manufacture, corrosion prevention and paint adhesion have to meet the strictest requirements because vehicles are exposed to the most serious corrosive stresses. At present, no vehicle bodies are made exclusively from organically precoated metal sheets. Instead, this material is made up into the vehicle bodies together with non-precoated sheets. Accordingly, the assembled bodies at present still go through the usual pretreatment process before painting, i.e. they are subjected to the expensive process of phosphating.

In principle, the phosphating process could be replaced by a less expensive pretreatment process if the vehicle bodies were to be made exclusively from organically precoated metal substrate. However, this would necessitate finding a solution to the problem that, in the assembly of bodies from organically precoated metal sheets, areas are inevitably formed where the organic precoating is damaged or missing altogether. This is the case, for example, at cut edges, spot welds or ground areas.

In the interests of a better corrosion-inhibiting effect, organically precoated metal substrates where electrolytically galvanized or hot dip galvanized steel is used as the metal substrate are frequently used in vehicle manufacture. However, with organically coated metal substrates such as these, the areas mentioned where the organic layer is damaged are particularly difficult to handle because they differ from the usual metal surfaces in regard to their electrochemical potentials and their chemical reactivity. In damaged places such as these, parts both of the steel substrate (i.e. iron) and of the zinc coating generally remain bare. A high local area ratio of steel (iron) to zinc, for example a ratio of >9:1, can be present. This is the case in particular with cut edges which represent a cross-section through the coated steel substrate. At these border areas which combine zinc and iron, the corrosion ratios differ from the other ratios on the homogeneous surface. Depending on the local ratio of zinc to iron in the exposed metal areas, a different electrochemical potential is established between the potentials of zinc and iron. In addition, ground areas with special ratios and hence particular electrochemical potentials are formed in the treatment of bodies because an activated interface between steel (iron) and fine-particle reactive zinc is formed by the grinding process.

The two-stage treatment process according to the invention is suitable for producing an adequate passivation layer for further painting in the above-mentioned problem areas where the organic precoating is damaged or missing altogether.

Accordingly, a special aspect of the present invention consists in a process for producing a structural element containing painted metal parts in which

-   -   I) sheets of metal carrying a coating based on organic polymers         are cut and/or stamped and/or formed and the metal parts         obtained are fitted together to form the structural element,         areas of the metal surface of the sheet which are not covered by         the coating based on organic polymers being formed;     -   II) the assembled structural element is cleaned,     -   III) the cleaned, assembled structural element is subjected to a         process sequence which produces a passivation layer that is not         a zinc phosphate layer on those areas of the metal surface         formed in step 1) which are not covered by the coating based on         organic polymers, the areas of the metal surface of the         structural element which are not covered by the coating based on         organic polymers     -   a) being contacted in a first step with a chromium-free first         aqueous solution having a pH of 1.5 to 5 which contains in all         at least 0.01 g/l, preferably at least 0.025 g/l and up to 10         g/l, preferably up to 1 g/l and more particularly up to 0.5 g/l         Ti and/or Zr and/or Si ions and at least such a quantity of         fluoride that the atomic ratio of Ti to F and/or Zr to F and/or         Si to F is in the range from 1:1 to 1:6, and then—optionally         after rinsing with water—     -   b) being contacted in a second step with a chromium-free second         aqueous solution having a pH of 1.5 to 5 which contains in all         at least 0.01 g/l, preferably at least 0.025 g/l and up to 10         g/l, preferably up to 1 g/l and more particularly up to 0.5 g/l         Ti and/or Zr and/or Si ions and at least such a quantity of         fluoride that the atomic ratio of Ti to F and/or Zr to F and/or         Si to F is in the range from 1:1 to 1:6 and which additionally         contains such a quantity of soluble anions of oxo acids of         molybdenum and/or tungsten with the oxidation number VI that the         total concentration of molybdenum and/or tungsten, expressed as         MoO₂ ²⁻ and/or WO₄ ²⁻, is in the range from 5 to 1500 mg/l,     -   IV) if desired, the structural element treated in step b) is         rinsed one or more times with water, although this is not         essential, and     -   V) is coated with a layer of paint.

The following observations apply both to the general aspect of the present invention directed to a two-step process for the corrosion prevention treatment of metal surfaces and to the special application of the invention, that is a process for producing a structural element containing painted metal parts as described in the preceding paragraphs.

In contrast to the second aqueous solution which is used in step b), the first aqueous solution used in step a) preferably does not contain any compounds of molybdenum or tungsten. Its use does not afford any technical advantage here and would therefore be uneconomical. However, it would not be technically disadvantageous.

An aqueous treatment solution which contains in all at least 0.01 g/l, preferably at least 0.025 g/l and up to 10 g/l, preferably up to 1 g/l and more particularly up to 0.5 g/l Ti and/or Zr and/or Si ions and at least such a quantity of fluoride that the atomic ratio of Ti to F and/or Zr to F and/or Si to F is in the range from 1:1 to 1:6 and which may, but does not have to, additionally contain at least 0.005 g/l, preferably at least 0.01 g/l and up to 20 g/l, preferably up to 1 g/l, organic polymers is used both in step a) and in step b).

The Ti, Zr and/or Si ions mentioned may be completely used in the form of hexafluoro complexes such as, for example, the hexafluoro acids or their salts soluble in water in the concentration range mentioned, such as the sodium salts for example. In this case, the atomic ratio is 1:6. However, complex compounds where less than six fluoride ions are attached to the central elements Ti, Zr or Si may also be used. These may be spontaneously formed in the treatment solution if both hexafluoro complexes of at least one of the central elements Ti, Zr or Si and at least one other compound of one of these central elements are added to the treatment solution. Suitable other compounds are, for example, nitrates, carbonates, hydroxides and/or oxides of the same central element or of another of the three central elements mentioned. For example, the treatment solution may contain hexafluorozirconate ions and (preferably colloidal) silica (SiO₂) or reaction products thereof. Unreacted silica may be suspended in the treatment solution. A treatment solution such as this may also be obtained by using hydrofluoric acid or (optionally acidic) salts thereof together with compounds of Ti, Zr and/or Si which are capable of forming fluoro complexes therewith. Examples are the already mentioned nitrates, carbonates, hydroxides and/or oxides. A preferred embodiment is characterized in that, in all, such a quantity of Ti, Zr and/or Si as the central metal and such a quantity of fluoride are used that the atomic ratio of central metal to fluoride is 1:2 or lower, more particularly 1:3 or lower. The atomic ratio may even be lower than 1:6 if the treatment solution contains more fluoride, for example in the form of hydrofluoric acid or salts thereof, than is stoichiometrically necessary for forming the hexafluoro complexes of the central metals Ti, Zr and/or Si. For example, the atomic ratio may be as low as 1:12 or 1:18 or even lower if a corresponding excess of fluoride is used, i.e. two, three or even more times the quantity required for complete formation of the hexafluoro complexes.

Treatment solutions which contain known combinations of ingredients may be used in step a). By way of non-limiting example, the following treatment solutions are considered suitable:

a treatment solution according to U.S. Pat. No. 5,129,967 containing at least the following components in water:

-   -   a) polyacrylic acid or homopolymers thereof,     -   b) hexafluorozirconic acid,     -   c) 0.17 to 0.3 g/l hydrofluoric acid and     -   d) up to 0.6 g/l hexafluorotitanic acid,

a treatment solution according to EP-0 008 942 B1 containing

-   -   a) polyacrylic acid or an ester thereof and     -   b) at least one of the compounds H₂ZrF₆, H₂TiF₆ and H₂SiF₆, the         pH of the solution being below 3.5,     -   (other polymers which may be used in similar treatment baths are         mentioned in WO 02/20652),

a treatment solution according to US-A-4,992,116 having pH values of about 2.5 to 5 which contains at least three components:

-   -   a) phosphate ions in a concentration of 1.1×10⁻⁵ to 5.3×10⁻³         mol/l, corresponding to 1 to 500 mg/l,     -   b) at least one fluoro acid of an element of the group         consisting of Zr, Ti and Si and     -   c) a polyphenol compound obtainable by reaction of         poly(vinylphenol) with aldehydes and organic amines,

a treatment solution according to WO 92/07973 which contains H₂ZrF₆ and a 3-(N—C₁₄-alkyl-N-2-hydroxyethylaminomethyl)-4-hydroxystyrene polymer as essential components in an acidic aqueous solution.

Treatment solutions where the organic polymers are selected from homo- and copolymers of vinyl pyrrolidone may also be used in step a). Such treatment solutions are described in DE-A-100 05 113 and DE-A-101 31 723. If, therefore, a treatment solution containing copolymers of vinyl pyrrolidone is used in the process according to the invention, these copolymers may contain one or more other monomers besides vinyl pyrrolidone. Accordingly, they may be present, for example, as copolymers of two components or as copolymers of three components (=terpolymers). In addition, mixtures of homopolymers and two-component copolymers, homo- and terpolymers or two-component copolymers and terpolymers may be used.

Suitable homo- or copolymers of vinyl pyrrolidone are, for example, the polymers listed in Table A or polymers of the monomers mentioned in Table A. Copolymers of vinyl pyrrolidone with monomers containing caprolactam or imidazole groups are particularly preferred. TABLE A Examples of homo- or copolymers of vinyl pyrrolidone Trade name or Name manufacturer Vinyl pyrrolidone, homopolymer Luviskol ®, BASF/ISP Vinyl pyrrolidone/vinyl acetate Luviskol ®, BASF/ISP Vinyl pyrrolidone/vinyl caprolactam Luvitec ®, BASF Vinyl pyrrolidone/vinyl imidazole Luvitec ®, BASF, Sokalan ® HP 56, BASF Vinyl pyrrolidone/vinyl imidazolium Luvitec ®, BASF methyl sulfate Vinyl pyrrolidone/Na methacrylate Luvitec ®, BASF Vinyl pyrrolidone/olefins ISP ®, Antaron Vinyl pyrrolidone/dimethylaminoethyl ISP ® methacrylate Vinyl pyrrolidone/dimethylaminopropyl ISP ®, Styleze methacrylamide Vinyl pyrrolidone/dimethylaminoethyl ISP ®, Gafquat methacrylate ammonium salt Vinyl pyrrolidone/vinyl caprolactam/ ISP ® dimethylaminoethyl methacrylate Vinyl pyrrolidone/methacrylamidopropyl ISP ®, Gafquat trimethyl ammonium chloride Vinyl pyrrolidone/vinyl caprolactam/ ISP ®, Advantage dimethylaminoethyl methacrylate Vinyl pyrrolidone/styrene ISP ®, Antara

If the treatment solution for step a) contains organic polymers, they are preferably present in concentrations of at least 0.005 g/l, more particularly at least 0.01 g/l and up to 20 g/l, more particularly up to 1 g/l.

By contrast, the treatment solution for step b) is preferably free from organic polymers. Although their use in step b) is not attended by any disadvantages, it does not afford any significant technical advantage either and is therefore uneconomical.

In step b), the words “soluble anions of oxo acids of molybdenum and/or tungsten with the oxidation number VI” mean that the compounds, generally salts containing the anions mentioned, are soluble under the conditions described herein in regard to concentrations, pH values and temperatures in the treatment solution. In this connection, the expert is aware that, under the conditions mentioned in the treatment solution, the anions of the oxo acids of molybdenum and/or tungsten with the oxidation number VI may easily be present in a form different from that in which they were introduced. A critical factor in this regard is, firstly, the pH-dependent protolysis equilibria of the respective anions. Accordingly, they may be partly present as free anions, but also in partly or completely protonated form, depending on the pH value. Secondly, the expert is aware that the anions mentioned have a tendency towards condensation or hydrolysis, depending on temperature, pH and concentration. Here, too, the position of the corresponding equilibria is critical. For example, the anions of the oxo acids of molybdenum and/or tungsten with the oxidation number VI may be introduced in the form of the orthometallates, metametallates, parametallates, polymetallates, such as in particular heptametallates, or as heteropolymetallates. “Metallates” in this context are understood to be the molybdates or the tungstates. They are preferably used as salts with cations such as, for example, sodium, potassium, lithium, calcium, cerium, barium, magnesium, strontium, ammonium, or as corresponding acids.

The metal surfaces which may be treated by the process according to the invention are preferably selected from surfaces of steel, galvanized or alloy-galvanized steel, aluminized steel, zinc, aluminum, magnesium or alloys of which at least 50 atom-% consist of zinc, aluminum or magnesium. Different metal surfaces may also be present alongside one another, as is increasingly the case in car manufacture. The process according to the invention shows its advantages in particular in the treatment of steel. For these surfaces in particular, there has never been a suitable process apart from phosphating for meeting the stringent anti-corrosion and paint adhesion requirements in car manufacture. When applied to steel surfaces, however, the process according to the invention leads to anti-corrosion and paint adhesion properties comparable with those of technically advanced phosphating.

Both the first and the second treatment solution may contain metal ions which have been dissolved out from the surfaces to be treated. These metal ions are, in particular, iron, zinc and/or aluminum ions which may be present, for example, in concentrations of 0.001 to 1 g/l and, more particularly, in concentrations of 0.005 to 0.5 g/l.

The second aqueous solution may additionally contain complexing agents, for example in quantities of 0.01 to 10 g/l and more particularly 0.05 to 5 g/l. With smaller quantities, the technical advantage is increasingly lost. Larger quantities do not afford any additional advantage and are therefore uneconomical. However, they are also not problematic. The complexing agents may be selected, for example, from chelating anions of hydroxycarboxylic acids or polybasic carboxylic acids such as, for example, lactate, oxalate, citrate, tartrate or gluconate. Acetyl acetonate is another suitable complexing agent. Other suitable complexing agents are amino-, imino- or nitrilocarboxylic acids, such as for example ethylenediamine tetra-acetic acid or nitrilotriacetic acid or anions thereof. Polybasic phosphonates, phosphonocarboxylates or amino-, imino- or nitriloalkylene phosphonates are also suitable complexing agents. Examples include 1 -hydroxyethane-1,1-diphosphonic acid, nitrilotri(methylenephosphonic acid), phosphonobutanetricarboxylic acid and other phosphonic acids with similar properties known to the expert.

The first and/or the second treatment solution preferably has a pH of at least 1.5, more particularly of at least 1.7, and up to 5, preferably up to 4. The pH values of the first and second treatment solutions may of course be different. In a particularly preferred embodiment, the second aqueous solution contains buffer substances for the pH range mentioned.

As mentioned above, the presence of organic polymers in the second aqueous solution does not afford any significant technical advantage. Accordingly, it is preferred for economic reasons if the second aqueous solution does not contain more than 5 mg/l organic polymers.

Step a) may be carried out using the treatment parameters mentioned in the cited literature. This applies in particular to the temperature of the first aqueous solution, to the contact time with the metal surface and to the method of application. In the second step b), the metal surfaces are preferably contacted with the aqueous solution for 5 seconds to 10 minutes and more particularly for 10 seconds to 1 minute. Contacting may be carried out as usual by immersing the metal surfaces in the treatment solution, by spraying them with the treatment solution or by combinations thereof. The temperature of the second aqueous solution is preferably at least 20° C., more particularly at least 30° C., and preferably no more than 60° C. and more particularly no more than 50° C. If the temperatures are any lower, the effect of the treatment increasingly diminishes. Although temperatures above the upper limits mentioned are not harmful, they are unnecessary and, hence, uneconomical in view of the increased energy demand.

EXAMPLES

Abbreviations:

s=seconds; mins=minutes; h=hours; d=days

DE water=deionized water; CASS=copper-accelerated salt spray test

EG=electrolytically galvanized steel, CRS=cold-rolled steel

CTL=cathodic dipping paint

-   -   Ridoline® and Ridosol® are alkaline cleaners from Henkel KGaA.     -   Sokalan® HP 56 is a vinyl imidazole/vinyl pyrrolidone copolymer         (BASF), CAS No.117197-37-2.     -   Molybdate was introduced in the form of the ammonium salt.     -   Where necessary, pH values were adjusted downwards with nitric         acid and upwards with ammonia solution or with sodium carbonate         solution.

Example 1

MoO₄/ZrF₆ rinse after conversion treatment with H₂ZrF₆/Sokalan HP56

Substrate: aluminum AA 6016

Process step sequence (spray application)

-   -   1. Cleaning: Ridoline 7163 CF, 1.5%; Ridosol 550 CF, 0.15%; 80         s; 55° C.     -   2. Rinse: industrial water     -   3. Rinse: DE water     -   4. Conversion treatment: 108 s; 30° C.; pH 3.8 bath composition:         -   H₂ZrF₆ acid (corresponding to 150 mg/l Zr) and         -   Sokalan HP 56 (corresponding to 40 mg/l solids)     -   5. Reactive rinse: 40 s, 30° C.; pH 1.8         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 50 mg/l Zr) and         -   molybdate (corresponding to 500 mg/l MoO₄ ²⁻)     -   6. Rinse: DE water     -   7. Drying: compressed air     -   8. Painting: CDL Cathoguard 310 (BASF); layer thickness ca. 20         μm         Corrosion Test:

Aluminum, DIN 50021 CASS, 10 d; creepage at scratch

Comparison Example 1

Conversion treatment with H₂ZrF₆/Sokalan HP56, no post-rinse

Substrate: aluminum AA 6016

Process step sequence (spray application)

-   -   1. Cleaning: Ridoline 7163 CF, 1.5%; Ridosol 550 CF, 0.15%; 80         s; 55° C.     -   2. Rinse: industrial water     -   3. Rinse: DE water     -   4. Conversion treatment: 108 s; 30° C.; pH 3.8         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 150 mg/l Zr) and         -   Sokalan HP 56 (corresponding to 40 mg/l)     -   5. Rinse: DE water     -   6. Drying: compressed air     -   7. Painting: CDL Cathoguard 310 (BASF); layer thickness ca. 20         μm         Corrosion Test:

Aluminum, DIN 50021 CASS, 10 d; creepage at scratch TABLE 1 Results of test series 1 Creepage, DIN 50021 CASS 10 d Example 1 0.8 mm Comparison Example 1 1.3 mm

Example 2

MoO₄/ZrF₆ rinse after conversion treatment with H₂ZrF₆/Sokalan HP56

Substrate: EG

Process step sequence (dip application)

-   -   1. Cleaning: Ridoline 1570, 3%; Ridosol 1237, 0.3%; 7 mins.; 55°         C.     -   2. Rinse: industrial water     -   3. Rinse: DE water     -   4. Conversion treatment: 180 s; 30° C.; pH 3.8         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 150 mg/l) and         -   Sokalan HP 56 (corresponding to 37 mg/l solids content)     -   5. Rinse: DE water     -   6. Reactive rinse: 30 s, 30° C.; pH 1.8         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 50 mg/l Zr) and         -   molybdate (corresponding to 500 mg/[ MoO₄ ²⁻)     -   7. Rinse: DE water     -   8. Drying: compressed air     -   9. Painting: CDL Cathoguard 310 (BASF); layer thickness ca. 20         μm

Comparison Example 2

Conversion treatment with H₂ZrF₆/Sokalan HP56, no post-rinse

Substrate: EG

Process step sequence (dip application)

-   -   1. Cleaning: Ridoline 1570, 3%; Ridosol 1237, 0.3%; 7 mins.; 55°         C.     -   2. Rinse: industrial water     -   3. Rinse: DE water     -   4. Conversion treatment: 180 s; 30° C.; pH 3.8         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 150 mg/l) and         -   Sokalan HP 56 (corresponding to 37 mg/l solids content)     -   5. Rinse: DE water     -   6. Drying: compressed air     -   7. Painting: CDL Cathoguard 310 (BASF); layer thickness ca. 20         μm         Paint Adhesion Test:

EG Salt water storage test (5% NaCI solution, 40° C., 6 d; chipping to VDA 621-427 before and after) TABLE 2 Results of test series 2 Chipping (K value, 1-10) 1 = very good, 10 = unacceptable Example 2 K 2.5 Comp. Example 2 K 3.5

Examples 3a and 3b

MoO₄/ZrF₆ rinse after conversion treatment with H₂ZrF₆/Sokalan HP56

Substrate: EG

Process step sequence (dip application)

-   -   1. Cleaning: Ridoline 1570, 3%; Ridosol 1237, 0.3%; 7 mins.; 55°         C.     -   2. Rinse: industrial water     -   3. Rinse: DE water     -   4. Conversion treatment: 180 s; 30° C.; pH 3.8         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 150 mg/l) and         -   Sokalan HP 56 (corresponding to 37 mg/l solids content)     -   5. Rinse: DE water     -   6. Reactive rinse: 60 s, 30° C.; pH 1.8         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 50 mg/l Zr) and         -   MoO₄ ²⁻ as per Table (Examples 3a and 3b)     -   7. Rinse: DE water     -   8. Drying: compressed air     -   9. Painting: CDL Cathoguard 310 (BASF); layer thickness ca. 20         μm and full buildup (car quality)

Comparison Example 3a

Conversion treatment with H₂ZrF₆/Sokalan HP56, no post-rinse

Substrate: EG

Process step sequence (dip application)

-   -   1. Cleaning: Ridoline 1570, 3%; Ridosol 1237, 0.3%; 7 mins.; 55°         C.     -   2. Rinse: industrial water     -   3. Rinse: DE water     -   4. Conversion treatment: 180 s; 30° C.; pH 3.8         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 150 mg/l) and         -   Sokalan HP 56 (corresponding to 37 mg/l solids content)     -   5. Rinse: DE water     -   6. Drying: compressed air     -   7. Painting: CDL Cathoguard 310 (BASF); layer thickness ca. 20         μm and full buildup (car quality)

Comparison Examples 3b and 3c

MoO₄ rinse (without ZrF₆) after conversion treatment with H₂ZrF₆/Sokalan HP56

-   -   1. Cleaning: Ridoline 1570, 3%; Ridosol 1237, 0.3%; 7 mins.; 55°         C.     -   2. Rinse: industrial water     -   3. Rinse: DE water     -   4. Conversion treatment: 180 s; 30° C.; pH 3.8         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 150 mg/l) and         -   Sokalan HP 56 (corresponding to 37 mg/l solids content)     -   5. Rinse: DE water     -   6. Reactive rinse: 60 s, 30° C.; pH 1.8         -   bath composition:         -   MoO₄ ²⁻ as per Table 3     -   7. Rinse: DE water     -   8. Drying: compressed air     -   9. Painting: CDL Cathoguard 310 (BASF); layer thickness ca. 20         μm and full buildup (car quality)         Paint Adhesion Tests:

EG Alternating climate test VDA 621415, 70 d; chipping to VDA 621-427 before and after TABLE 3 Results of test series 3 Chipping (K value, 1-10) MoO₄ ²⁻ 1 = very good, in mg/l 10 = unacceptable Example 3a 100 K 2.0 Example 3b 500 K 3.0 Comp. Example 3a — K 3.7 Comp. Example 3b 100 K 9.7 Comp. Example 3c 500 K 10.0

Example 4

MoO₄/ZrF₆ rinse with different pH values after conversion treatment with H₂ZrF₆/Sokalan HP56

Substrate: EG

Process step sequence (spray application)

-   -   1. Cleaning: Ridoline 7163 CF, 1.5%; Ridosol 550 CF, 0.15%; 80         s; 55° C.     -   2. Rinse: industrial water     -   3. Rinse: DE water     -   4. Conversion treatment: 108 s; 30° C.; pH 3.8         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 150 mg/l Zr) and         -   Sokalan HP 56 (corresponding to 37 mg/l solids)     -   5. Reactive rinse: 40 s, 30° C.;         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 50 mg/l Zr) and         -   molybdate (corresponding to 500 mg/l MoO₄ ²⁻); pH values             adjusted as shown in Table 4     -   6. Rinse: DE water     -   7. Drying: compressed air     -   8. Painting: CDL Cathoguard 310 (BASF); layer thickness ca. 20         μm         Paint Adhesion Test:

EG Salt water storage test (5% NaCI solution, 40° C., 6 d; chipping to VDA 621-427 before and after)

Comparison Example 4

Conversion treatment with H₂ZrF₆/Sokalan HP56, no post-rinse

Substrate: EG

Process step sequence (spray application)

-   -   1. Cleaning: Ridoline 7163 CF, 1.5%; Ridosol 550 CF, 0.15%; 80         s; 55° C.     -   2. Rinse: industrial water     -   3. Rinse: DE water     -   4. Conversion treatment: 108 s; 30° C.;         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 150 mg/l Zr) and         -   Sokalan HP 56 (corresponding to 40 mg/l)     -   5. Rinse: DE water     -   6. Drying: compressed air     -   7. Painting: CDL Cathoguard 310 (BASF); layer thickness ca. 20         μm         Paint Adhesion Test:

EG Salt water storage test (5% NaCI solution, 40° C., 6 d; chipping to VDA 621-427 before and after) TABLE 4 Results of test series 4 Chipping (K value, 1-10) 1 = very good, pH 10 = unacceptable Example 4a 2.8 K 7.3 Example 4b 2.4 K 7.2 Example 4c 2.0 K 6.3 Comp. Example 4 — K 9.5

Example 5

Process used as post-rinse after conversion treatment

Substrate: CRS

Process step sequence (dip application)

-   -   1. Cleaning: Ridoline 1570, 2%; Ridosol 1237, 0.3%; 5 mins.; 55°         C.     -   2. Rinse: water     -   3. Rinse: DE water     -   4. Conversion treatment: 180 s; 30° C.; pH 4.0         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 150 mg/l Zr) and 40 mg/l             polymer (phenol/salicylic acid/formaldehyde resin grafted             with 0.5 part imidazole, based on phenol (ratio of phenol to             salicylic acid 1:1))     -   5. Rinse: DE water     -   6. Reactive rinse: 60 s, 30° C.;         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 50 mg/l Zr) and         -   molybdate (corresponding to 500 mg/l MoO₄-²⁻); pH 2.8     -   7. Rinse: DE water     -   8. Drying: compressed air     -   9. Painting: polyester PES 5807/RAL 5009 GL (TIGC-free) from         IGP; ca. 60-80 μm

Comparison Example 5a

Conversion process as 5 with no reactive post-rinse

Substrate: CRS

Process step sequence (dip application)

-   -   1. Cleaning: Ridoline 1570, 2%; Ridosol 1237, 0.3%; 5 mins.; 55°         C.     -   2. Rinse: water     -   3. Rinse: DE water     -   4. Conversion treatment: 180 s; 30° C.; pH 3.0         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 150 mg/l Zr) and 40 mg/l             polymer (phenol/salicylic acid/formaldehyde resin grafted             with 0.5 part imidazole, based on phenol (ratio of phenol to             salicylic acid 1:1))     -   5. Rinse: DE water     -   6. Drying: compressed air     -   7. Painting: polyester PES 5807/RAL 5009 GL (TIGC-free) from         IGP; ca. 60-80 μm

Comparison Example 5b

Post-rinse from 5 used as a conversion bath

Substrate: CRS

Process step sequence (dip application)

-   -   1. Cleaning: Ridoline 1570, 2%; Ridosol 1237, 0.3%; 5 mins.; 55°         C.     -   2. Rinse: water     -   3. Rinse: DE water     -   4. Conversion treatment: 180 s; 30° C.;         -   bath composition:         -   H₂ZrF₆ acid (corresponding to 50 mg/l Zr) and         -   molybdate (corresponding to 500 mg/l MoO₄ ²⁻); pH 2.8     -   5. Rinse: DE water     -   6. Drying: compressed air

7. Painting: polyester PES 5807/RAL 5009 GL (TlGC-free) from IGP; ca. 60-80 μm TABLE 5 Corrosion prevention Test (DIN 50021 SS, 21 d) U/2 in mm) Example 5 2.3 Comp. Example 5a 2.9 Comp. Example 5b >15 

1. A process for the two-step corrosion prevention treatment of metal surfaces, comprising: a) contacting a metal surface with a chromium-free, first aqueous solution having a pH of 1.5 to 5 which contains 0.01 g/l to 10 g/l of one or more first metal ions selected from Ti, Zr and Si ions, and at least a quantity of fluoride such that the atomic ratio of the first metal ions to the fluoride is in the range from 1:1 to 1:6 and then, optionally after rinsing with water, b) contacting the metal surface from a), with a chromium-free, second aqueous solution having a pH of 1.5 to 5 which contains 0.01 g/l to 10 g/l of said one or more first metal ions and at least a quantity of fluoride such that, in the second aqueous solution, the atomic ratio of the first metal ions to the fluoride is in the range from 1:1 to 1:6, said second aqueous solution additionally comprising a quantity of soluble anions of oxo acids of Mo (VI) and/or W (VI), such that the total concentration of molybdenum and/or tungsten, expressed as MoO₂ ²⁻ and/or WO₄ ²⁻, is in the range from 5 to 1500 mg/l.
 2. The process as claimed in claim 1, wherein metal surfaces are selected from surfaces of steel, galvanized or alloy-galvanized steel, aluminized steel, zinc, aluminum, magnesium or alloys of which at least 50 atom-% consists of zinc, aluminum or magnesium.
 3. The process as claimed in claim 1, wherein the first aqueous solution additionally contains at least 0.005 g/l and up to 20 g/l organic polymers.
 4. The process as claimed in claim 3, wherein the first aqueous solution contains at least 0.01 g/l and up to 1 g/l organic polymers.
 5. The process as claimed in claim 1, wherein the second aqueous solution additionally contains complexing agents.
 6. The process as claimed in claim 1, wherein the first and/or the second aqueous solution has a pH of at least 1.7 and up to
 4. 7. The process as claimed in claim 1, wherein the second aqueous solution additionally contains buffer substances for the pH range of 1.5 to
 5. 8. The process as claimed in claim 1, wherein the second aqueous solution contains no more than 5 mg/l organic polymers.
 9. The process as claimed in claim 1, wherein, in the second step b), the metal surfaces are contacted with the second aqueous solution for 5 seconds to 10 minutes.
 10. The process as claimed in claim 1, wherein the second aqueous solution has a temperature of at least 20° C. and no more than 60° C.
 11. A process for producing a structural element containing painted metal parts in which I) cutting and/or stamping and/or forming sheets of metal carrying a coating based on organic polymers into metal parts and fitting together the metal parts obtained to form a structural element, areas of metal surface which are not covered by the coating based on organic polymers being produced thereby; II) cleaning the assembled structural element, III) subjecting the cleaned, assembled structural element to a process sequence which produces a passivation layer that is not a zinc phosphate layer on said areas of the metal parts which are not covered by the coating based on organic polymers formed in step I), said process sequence comprising: a) contacting the cleaned, assembled structural element in a first step with a chromium-free first aqueous solution having a pH of 1.5 to 5 which contains at least 0.01 g/l and up to 10 g/l Ti and/or Zr and/or Si ions and at least a quantity of fluoride such that the atomic ratio of Ti to F and/or Zr to F and/or Si to F is in the range from 1:1 to 1:6, and then, optionally after rinsing with water; b) contacting the cleaned, assembled structural element in a second step with a chromium-free second aqueous solution having a pH of 1.5 to 5 which contains at least 0.01 g/l and up to 10 g/l Ti and/or Zr and/or Si ions and at least such a quantity of fluoride that the atomic ratio of Ti to F and/or Zr to F and/or Si to F is in the range from 1:1 to 1:6 and said second aqueous solution additionally comprising a quantity of soluble anions of oxo acids of Mo (VI) and/or W (Vl), such that the total concentration of molybdenum and/or tungsten, expressed as MoO₂ ²⁻ and/or WO₄ ²⁻, is in the range from 5 to 1500 mg/l; IV) optionally, rinsing the structural element treated in step b) one or more times with water, and V) coating the treated structural element with a layer of paint.
 12. The process as claimed in claim 11, wherein the metal surfaces are selected from surfaces of steel, galvanized or alloy-galvanized steel, aluminized steel, zinc, aluminum, magnesium or alloys of which at least 50 atom-% consists of zinc, aluminum or magnesium.
 13. The process as claimed in claim 11, wherein the first aqueous solution additionally contains at least 0.005 g/l and up to 20 g/l organic polymers.
 14. The process as claimed in claim 13, wherein the first aqueous solution contains at least 0.01 g/l and up to 1 g/l organic polymers.
 15. The process as claimed in claim 11, wherein the second aqueous solution additionally contains complexing agents.
 16. The process as claimed in claim 11, wherein the first and/or the second aqueous solution has a pH of at least 1.7 and up to
 4. 17. The process as claimed in claim 11, wherein the second aqueous solution additionally contains buffer substances for the pH range of 1.5 to
 5. 18. The process as claimed in claim 11, wherein the second aqueous solution contains no more than 5 mg/l organic polymers.
 19. The process as claimed in claim 11, wherein, in the second step b), the metal surfaces are contacted with the second aqueous solution for 5 seconds to 10 minutes.
 20. The process as claimed in claim 11, wherein the second aqueous solution has a temperature of at least 20° C. and no more than 60° C. 